1. Field of the Invention
The present invention relates to a method and apparatus for data transmission in a network supporting coordinated transmission. More particularly, the present invention relates to a data transmission method and apparatus wherein, in order to send data to a User Equipment (UE) through multiple transmission points, a base station sends information used to generate a Demodulation Reference Signal (DMRS) scrambling sequence and the UE generates the DMRS scrambling sequence using the received information.
2. Description of the Related Art
In contrast to related-art mobile communication systems that may provide only voice-oriented services, advanced mobile communication systems may provide high-quality data and multimedia services using high-speed packet data communication. In recent years, in order to support high-speed and high-quality packet data transmission services, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), Long Term Evolution Advanced (LTE-A), developed by the 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD), developed by the 3GPP 2 (3GPP2), and Institute of Electrical and Electronics Engineers (IEEE) 802.16, have been developed.
In particular, the LTE system has been developed so as to efficiently support high-speed wireless packet data transmission and also attempts to maximize wireless system throughput using various radio access technologies. LTE-A is an advanced version of LTE with increased data rates. Existing 3rd Generation (3G) packet data communication systems, such as HSDPA, HSUPA and HRPD employ Adaptive Modulation and Coding (AMC) and channel-aware scheduling in order to enhance transmission efficiency.
A transmitter using AMC may adjust an amount of transmission data according to channel state. That is, when channel conditions are not good, the amount of transmission data may be reduced so as to maintain a desired error rate at the receiver, and when channel conditions are favorable, the amount of transmission data may be increased for higher efficiency while maintaining a desired error rate at the receiver.
The transmitter using channel-aware scheduling for resource management may selectively serve a specific user, from among many users, with the most favorable channel conditions. Hence, system throughput may be increased in comparison to a case in which channel resources are allocated to one user. This effect is referred to as multi-user diversity gain. In other words, the transmitter employing AMC and channel-aware scheduling receives partial channel state information as feedback from receivers and applies appropriate modulation and coding schemes in a timely manner.
When AMC is used together with Multiple Input Multiple Output (MIMO) transmission, AMC may also be used to determine a number, or rank, of spatial layers, which may also be referred to as transmission layers, for signal transmission. In this case, in addition to the coding rate and modulation scheme, the number of transmission layers to be used may be considered in order to determine an optimal data rate.
Recently, Code Division Multiple Access (CDMA) used in 2nd Generation (2G) and 3G mobile communication systems is being merged into Orthogonal Frequency Division Multiple Access (OFDMA) in next generation mobile communication systems. As OFDMA is expected to increase system throughput beyond CDMA, systems developed by the 3GPP and 3GPP2 have initiated standardization of evolved systems based on OFDMA. Increased system throughput using OFDMA may be achieved by using frequency domain scheduling. As channel-aware scheduling considers channel conditions varying with time in order to increase system throughput, consideration of channel conditions varying with frequency may contribute to further enhancement of system throughput.
FIG. 1 illustrates a cellular system having multiple cells according to the related art.
Referring to FIG. 1, a cellular system may provide mobile communication services using various schemes described above. More specifically, in the mobile communication system having three cells, Cell 0 through Cell 2 shown in FIG. 1, an antenna for transmission and reception is installed in each cell. A Base Station (BS), such as an enhanced Node B (eNB), may be placed at each of cells Cell 0, Cell 1 and Cell 2, in order to send data to User Equipments (UEs within the corresponding cell.
A UE 0 within a service area of Cell 0 receives a data signal 100 from the eNB of Cell 0. Similarly, eNBs of Cell 1 and Cell 2 respectively send data signals 110 and 120 to UE 1 and UE 2 using the same time-frequency resources. Respective transmissions from Cell 0, Cell 1 and Cell 2 to UE 0, UE 1 and UE 2 correspond to non-Coordinated MultiPoint (non-CoMP) transmission, wherein radio resources of one cell are used only for UEs within the cell.
In FIG. 1, a UE within a cell may know an available time-frequency resource from among signals sent by the corresponding eNB in advance. For example, UE 0 may determine a location of a Cell-specific Reference Signal (CRS) and a number of OFDM symbols on a control channel in a time-frequency grid 130 formed by Cell 0 before reception of a Physical Downlink Shared Channel (PDSCH).
As shown in FIG. 1, PDSCH resources allocated to UE 0, UE 1 and UE 2 are different in the time-frequency grids 130, 140 and 150 formed respectively by Cell 0, Cell 1 and Cell 2. When non-CoMP transmission is used, a UE receives a signal from a fixed cell. For example, in FIG. 1, UE 0 receives a signal only from Cell 0, unless it is handed over to another cell through separate higher layer signaling.
In the time domain, an LTE and/or LTE-A downlink transmission, as shown in FIG. 1, may be split into a control region and a data region. The control region may be used to transmit control channels, such as a Physical Downlink Control Channel (PDCCH), a Physical HARQ Indicator Channel (PHICH), and a Physical Control Format Indicator Channel (PCFICH). In a subframe, the control region may correspond to one, two or three OFDM symbols from the beginning.
The data region may start at an OFDM symbol that is disposed immediately after the control region, and may be used for a PDSCH transmission. Because a subframe is composed of fixed number of OFDM symbols, the data region size may be determined by the control region size. In the LTE and/or LTE-A system, a UE may refer to control information carried by the PCFICH in order to know the control region size and in order to determine the data region size accordingly.
In FIG. 1, a signal sent by one cell may interfere with a signal sent by another cell, and accordingly, randomization of interference may enhance signal reception performance. For example, when signals are respectively sent from Cell 0 and Cell 1 to UE 0 and UE 1 through the same radio resources, the signals may interfere with each other. Thus, it is desirable to randomize interference for better reception performance. For this reason, in the LTE and/or LTE-A system, different scrambling sequences are applied to DeModulation Reference Signals (DMRS) sent by different cells. In order to achieve this, scrambling sequence generators of different cells have different initial states, because scrambling sequence generators with different initial states generate different scrambling sequences. That is, when cells apply differently initialized scrambling sequences, inter-cell interference may be effectively randomized.
In contrast to non-CoMP transmission, CoMP transmission enables multiple base stations to send signals to one UE. When CoMP transmission is used, one UE may receive signals from multiple base stations. Hence, it is possible to provide an improved data rate to a UE far from a base station. Similarly to the case of non-CoMP transmission, in order to randomize interference between signals from multiple base stations participating in CoMP transmission, the multiple base stations should apply different scrambling sequences.
Therefore, a need exists for a system and method for multiple base stations to apply different scrambling sequences for CoMP transmissions.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.